This study is designed to elucidate the relationship between the structure of transposable elements and the mechanisms by which they join unrelated segments of DNA. We are investigating three systems: (1) Gamma:: Tn9 - Gamma phages containing the chloramphenicol resistance element Tn9 will insert into the bacterial chromosome in the absence of the normal viral integration system. Previous results showed that insertion and excision of Gamma:: Tn9 prophages are precise events invloving recombination at lSl elements at the prophage ends. We will examine the products of prophage excision to determine the nature of the excision reaction. (2) Artifical transposons constructed by recombination - A galE gene flanked by two lSl elements is transposable. The two lSl elements can be in parallel or opposed orientation. We will study the behavior of lSl - galE - lSl trasposons to determine the importance of direct versus opposed terminal repeats on insertion and excision events. (3) Tn5 - Substitution of trp DNA for certain parts of the Tn5 kanamycin-resistance transposon results in a recessive transposition defect. We will use in vivo and in vitro Tn5 mutants to determine what parts of the transposon encode diffusible activities required for transposition.